1. Field of the Invention
This invention is in the field of calcining in a vertical shaft furnace, and utilizing a recuperation zone intermediate the other zones of the furnace to carry out the calcining operation more efficiently from the standpoint of heat usage.
2. Description of the Prior Art
It is known from German Laid Open Pat. No. 2,364,626 to preheat the entire combustion air for a shaft furnace for the calcining of lime, dolomite, magnesite, or other carbon containing substances by contacting the same in a cooling zone with the lime which has been calcined to completion. In this arrangement, the portion of the air which remains in the shaft furnace flows through the bottom upwardly in countercurrent contact with the treatment material. In the known method, however, there is a disadvantage that the combustion air required for the operation of a lime calcining furnace is preheatable only to about 500.degree. C by its utilization as cooling air for the calcined lime. Since the deacidification temperature of the lime material ranges usually between 800.degree. and 900.degree. C., additional energy must be supplied either by use of an increased amount of fuel during calcination, or by means of a further preheating of the combustion air.
There has been proposed a regenerative preheating of the combustion air, as for example in the periodical "Zement-Kalk-Gips" No. 5, 1970, pages 206 to 209. With this arrangement, however, there is the disadvantage that the entire amount of combustion air is not used for the cooling of the calcined lime, since part of the combustion air which is preheated by means of the exhaust gas heat is drawn up as fresh air. The result is a rather inefficient overall process.